C57BL/6 mice, along with other mouse genotypes, have served as models of human presbycusis. C57BL/6 mice suffer from progressively severe hearing impairments, attributable primarily to losses of both hair cells (sensory receptors) and spiral ganglion cells (afferent neurons). Their age-related hair cell degeneration in the cochlea begins in the basal turn and progresses to the apex.
Auditory nerves also undergo age-related changes. The mammalian cochlear nerve is mixed, being composed largely of afferent (sensory) fibers and a small population of efferent fibers. Because efferent nerves are primarily cholinergic, we used acetylcholinesterase (AChE) as a marker enzyme for determining both age-related changes in AChE activity of the efferent fibers and also for tracing degenerative patterns in efferent pathways within the organ of Corti.
Twelve C57BL/6 mice of the following ages were used: four young (23 days), four middle-aged (13 months), and four old (22.5 months) mice.